The present invention relates to a reinforced sandwich structure comprising stacked in height direction thereof at least one top sheet and at least one bottom sheet comprising a fibrous reinforcing material, which top and bottom sheet sandwich between them at least one core of a foamed material, which sandwich structure is impregnated with a resin material to connect the top sheet to a top face of the core and the bottom sheet to a bottom face of the core, as disclosed in the preamble of the first claim.
DE1704872 discloses a method for producing a sandwich panel comprising a core of a foamed plastic material sandwiched between a top and a bottom sheet of a fibrous reinforced polyester resin. The sandwich panel is produced by aligning a plurality of cores of a foamed plastic material in such a way that the longitudinal axes of adjacent blocks run parallel to each other. Between adjacent blocks, a cross plate is applied and the edges of the core running along the cross plates are chamfered. In the tope and bottom face of each block a plurality of a parallel grooves is applied. A layer of an adhesive is applied to the face of the blocks facing the bottom sheet and the facing side faces of adjacent blocks. Thereafter a layer of an adhesive is applied to the top face of the blocks facing the top sheet. It is alleged that the adhesive fills the grooves and the space between adjacent blocks.
The sandwich panel disclosed in DE1704872 however presents the disadvantage that the production of the sandwich panel is too time consuming and laborious. Moreover, since the top and bottom cover sheets are connected to the core by means of an adhesive layer only, the sandwich panel is susceptible to de-lamination.
WO95/20486 discloses a structural honeycomb sandwich construction comprising outer layers of plywood, metal, plastic, various composition boards, cements and laminations of any combination of these. The outer layers sandwich between them core elements which assume an overall openwork patter formed from natural wood veneer, veneer formed from wood fibers or shavings combined with synthetic resins, wood shavings into selected geometric shapes or formed from bamboo into rings. The core elements may be secured to each other and to the outer facings by means of a resin and may be reinforced by a fibrous reinforced resin.
However, the above disclosed prior art processes are non-continuous, laborious and time consuming. There is thus a need to a process for producing sandwich structures, which is economically feasible and permits mass production of sandwich structures in a substantially continuous process.
From PCT/BE03/00092 a sandwich panel is known which comprises a top and a bottom layer enclosing a core. The top and bottom layer each comprise a fibrous reinforcing material and are connected to each other and to the core by means of tufting using a tow or yarn of fibrous reinforcing material, after which the sandwich panel is impregnated with a thermosetting resin using pulltrusion and cured.
However, the stiffness of the known sandwich panels is determined by the stiffness provided by its components, in particular the core, the fibrous reinforcing material of the top and bottom layer, and the fibrous reinforcing material used for the tufting and the resin. In particular the bending stiffness is determined by formula 1:
  D  =                    t        1            ⁢              t        2            ⁢              E        1            ⁢                                    E            2                    ⁡                      (                          h              +              c                        )                          2                    4      ⁢              (                                            t              1                        ⁢                          E              1                                +                                    t              2                        ⁢                          E              2                                      )            t1, t2: thickness of top and bottom sheetE1, E2: Elasticiteitsmodulus of top and bottom sheeth: Total thickness of the sandwich panelc: thickness of the core
The deflection of the sandwich panel with simply supported ends under a uniform transverse load (i.e. loan P on the floor of a trailer) is described by formula 2:
  w  =                    5        ⁢                  Pa          3                            384        ⁢        Db              ⁡          [              1        +                              192            ⁢            cD                                5            ⁢                          a              2                        ⁢                                                            G                  e                                ⁡                                  (                                      h                    +                    c                                    )                                            2                                          ]      a: span lengthGc: Shear Modulus of the core Materialb: Width of the sandwich panel